Because people often under-report their consumption of cannabis due to the stigma surrounding the drug, Statistics Canada turned to one thing people can’t lie about — the waste that comes out of their body. Essentially, StatCan tested the wastewater flushed down toilets using a new technique called wastewater-based epidemiology (WBE).

The federal agency says they conducted their analysis one week a month at 15 wastewater treatment plants. Statistics Canada released a report on Thursday providing data on Halifax, Montreal, Toronto, Edmonton and Vancouver from March to August of this year.

The analysis found that Halifax consumed the most cannabis at 1,310 micrograms per person per week, more than any of the other major metropolitan areas included in the project.

Montreal had the second-highest amount with 976 micrograms per person per week, Toronto had 451 micrograms per person per week, Edmonton had 416 micrograms per person per week and Vancouver’s result was 288 micrograms per person per week.

The load of cannabis metabolites measured in micrograms per capita per city.

Statistics Canada

Statistics Canada has stressed that the data released on Thursday should be considered “preliminary and experimental” but that it will continue to refine its methodology.

StatCan found that people in the province consumed 27.1 grams of cannabis per person per year.

Alberta and British Columbia were close behind, with citizens in those provinces consuming 24.6 grams and 24.1 grams, respectively.

All three of the provinces were above the Canadian average, which StatCan pegged at 21.1 grams per person.

Measuring cannabis in wastewater

Whenever someone consumes cannabis or tetrahydrocannabinol (THC), the main psychoactive component of cannabis, their bodies process the cannabis into metabolites, which are later eliminated from the body and flushed into municipal wastewater systems.

By analyzing the concentrations of metabolites in wastewater, Statistics Canada was able to estimate the total mass of cannabis consumed in a region.

Roughly 8.4 million people use the facilities included in the pilot project.

Statistics Canada says using the WBE system is a good option for the future as it’s cost-effective and yields rapid results that can be applied to smaller areas in the country.

“It may also be used to estimate the size of the illegal cannabis market following legalization by allowing Statistics Canada to subtract the volume of legal cannabis retail sales from total consumption as measured through WBE,” the report released on Thursday found.

Moncton researchers hope project will help alleviate cannabis production shortages

But the idea isn’t a new one. Similar studies and programs in Europe have proven quite effective at measuring the levels of drug use in various cities, detecting everything from marijuana and methamphetamines to ecstasy and cocaine.

Last April, Swedish public broadcaster SVT reported that wastewater collected from city sewers in the country was being sent to a lab for analysis, with scientists measuring the presence of cocaine, marijuana and methamphetamines.

The technology was said to be highly precise, even revealing the approximate number of doses of cocaine and marijuana that residents of the city of Gävle consumed on Christmas Day.

Statistics Canada says it is modelling its approach on the European one, specifically the methodology used by the European SCORE network (Sewage analysis CORe group – Europe).